We used CNN Money's helpful millionaire calculator to estimate when you'll become a millionaire if you're able to contribute $500 to an investment account each month, assuming that you're starting from scratch with zero savings.

These differences speak to the power of compound interest, in which any interest earned accrues interest on itself, and a little money invested now can end up being more than a lot of money invested later. In short: If you want to become a millionaire, the earlier you start investing, the better.

This calculation doesn't account for the many variables that can affect your wealth over several decades, including windfalls, emergencies and rises or dips in the market. But it can give you a good idea as to whether or not you're saving enough to retire comfortably.

Of course, saving hundreds or thousands a month is an ambitious goal. Even $2,100 a year is more than most Americans can manage. But getting into the habit of saving any amount will be great for you in the long run, and if this inspires you to get started, the simplest way is to invest in your employer's 401(k) plan, a tax-advantaged retirement savings account. Next, consider alternate retirement savings accounts, such as a Roth IRA, traditional IRA and/or a health savings account.